The present disclosure herein relates to a cold cathode fluorescent lamp (CCFL) for illumination, and more particularly, to a highly efficient, long-lifespan CCFL improved in tube current, optical efficiency, brightness, and lifespan for being used as an illumination light source in addition to conventional use as a backlight of a liquid crystal display, a scanning light source of a facsimile, an eraser lamp of a copier, etc.
In the related art, cold cathode fluorescent lamps (CCFLs) are used as light sources such as backlights of liquid crystal displays, scanning light sources of facsimiles, and eraser lamps of copiers, and necessary brightness levels for such devices can be obtained by applying only a tube current of about 4 to 4 mA to the CCLFs. Such a CCFL includes cup-shaped electrodes provided at both ends of a glass tube, and a fluorescent layer formed by applying a fluorescent material to the inner surface of the glass tube. Rare gas such as neon gas, argon gas, and xenon gas is filled in the glass tube together with a small amount of mercury, and the glass tube is sealed. If a high voltage is applied to the cup-shaped electrodes provided at both sides of the glass tube, a small number of electrons ionize the rare gas sealed in the glass tube, and secondary electrons are emitted from the cup-shaped electrodes as the ionized rare gas collide with the cup-shaped electrodes (this is called a glow discharge). The secondary electrons collide with the mercury, and as a result, the mercury emits ultraviolet rays toward the fluorescent layer formed on the inner surface of the glass tube. Then, the fluorescent material of the fluorescent layer emits visible light. At this time, a tube current of about 4 mA to 5 mA flows in the glass tube. However, a tube current of 10 mA or higher is necessary to increase the brightness of the CCFL to a level necessary for illumination.
In the related art, electrodes of a CCFL are formed into a cup shape to increase inner areas of the electrodes necessary for electron emission. In addition, such electrodes are mainly formed of nickel (Ni) because nickel (Ni) has a relative low melting point and can be easily machined into a desired shape such as a cup shape. However, nickel (Ni) or nickel alloys have high work functions and high sputtering coefficients. For this reason, cup-shape electrodes are formed of an Nb—Ni alloy or Y—Ni alloy for increasing the sputtering resistance of the cup-shaped electrodes. However, the lifespan of such cup-shaped electrodes is short due to sputtering if a tube current of 10 mA or higher is applied to the electrodes. Sputtering causes excessive heat generation at electrodes and largely decreases luminous efficacy. In addition, since a sputtering layer is formed on an inner surface of a glass tube due to sputtering, it is difficult to obtain a brightness level necessary for illumination if electrodes are sputtered. That is, electrodes formed of nickel (Ni) or a nickel alloy are not suitable for a CCFL having a tube current of 5 mA or higher, and thus it is difficult to use a CCFL including cup-shaped nickel or nickel-alloy electrodes as an illumination light source.
Furthermore, in the related, since electrodes having large area are preferred, the sizes of the electrodes are excessively increased. Large electrodes occupy large spaces in glass tubes, and thus spaces for positive columns are reduced to decrease luminous efficacy and energy efficiency. Therefore, it is difficult to use CCFLs as illumination light sources.